松辽盆地西斜坡中—晚二叠世古沉积环境演化及油气地质意义

Paleosedimentary environment evolution during the Middle-Late Permian in the western slope of Songliao basin and its petroleum geological implications

大兴安岭南部—松辽盆地西斜坡晚二叠世古沉积环境一直以来备受争议,关于该地区中—晚二叠世古环境演化方面的研究也鲜见报道。本文采用元素地球化学和有机地球化学相结合的研究方法,对松辽盆地西斜坡蒙科地1井中—晚二叠世古沉积环境演化特征进行综合研究。结果表明,中二叠世为明显的海相沉积,沉积水体相对较深,期间可能存在多次干热—暖湿波动,其中中二叠世ZSI晚期—ZSII早期较为明显,水体性质体现为咸水—微咸水—半咸水,水体还原性—偏氧化性—还原性,气候干热—温湿—干热。晚二叠世早期为微咸水—半咸水海相沉积,至晚二叠世晚期转变为淡水—微咸水海陆过渡相沉积,期间存在两次明显的气候环境波动,与中二叠世哲斯组沉积中期气候环境变化相似,表现为水体变浅、咸度降低、还原性减弱,气候由干热转向温湿。晚二叠世以来气候逐渐向温暖湿热转变,水体逐渐变浅,可能与二叠纪—三叠纪之交气候变暖有关。至三叠纪、侏罗纪时期,区域处于构造抬升阶段,沉积环境由海陆过渡相环境转变为陆相沉积。结合TOC、古生产力指标分析得出,中二叠世ZSI晚期—ZSII2早期、晚二叠世LXI晚期、晚二叠世LXII中期、晚二叠世LXIII沉积期偏湿润的气候条件和低咸度、弱还原—偏氧化性的沉积水体环境,是形成厚度较大、富有机质烃源岩的有利的气候环境条件。

The paleosedimentary environment in the Late Permian in the western slope of Songliao basin, southern Great Xing’an Range has always been controversial and little studies were reported on the paleosedimentary environment evolution during the Middle-Late Permian in this area. In this paper, the evolution characteristics of paleosedimentary environment of the Middle-Late Permian sedimentary rocks from well MKD1, in the western slope of Songliao basin, are comprehensively analyzed by elemental and organic geochemistry methods. The results show that the Middle Permian strata was apparently deposited in a marine environment, in relatively deep water. There may have been several dry heat-warm humidity fluctuations during this period, especially from late ZSI and early ZSII2 period of the Middle Permian.The deposition water varied from salty to brackish, and to semi-saline, the environment ranged from reducing to partially oxidizing, while the climate fluctuated between dry heat to warm wet and to dry heat. In the early Late Permian, it was marine deposition with brackish and semi-saline water, and changed to marine-continental transitional deposition with fresh-brackish water in the late Late Permian. During this period, there were two apparent climatic and environmental fluctuations, which are similar to the climatic and environmental alternation in the middle period of the Zhesi Formation-that the water became shallow, the salinity decreased, the water reducibility weakened, and the climate changed from dry and hot to warm and wet. Since the Late Permian, the climate had gradually changed to warm and humid, and the water had gradually become shallower, which may be related to the climate warming during the Permian-Triassic transition.In the Triassic and Jurassic period, the region was in the stage of tectonic uplift, and the marine-continental transitional environment was transformed into continental sedimentation. Combined with TOC and paleoproductivity indicators, it is concluded that the warm and humid climate conditions, low salinity of the water and weak reducing-partial strong oxidizing environment in the late ZSI and early ZSII2 period of the Middle Permian, late LXI period, middle LXII period, and LXIII period of the Late Permian were the favorable climate and environmental conditions for the formation of thick source rocks with high organic matter abundance.